This invention relates to a method of forming an aluminum (Al) based pattern, adapted for such applications as the production of semiconductor devices. This invention relates particularly to a method whereby an improvement of resist selectivity, a reduction in particle pollution and a prevention of after-corrosion may be achieved in dry etching of an Al-based metallization layer.
As electrode metallization materials of a semiconductor device, Al-based materials are broadly used, such as, Al , Al--Si alloy consisting of Al with 1 to 2% of silicon being added thereto, and Al--Si--Cu alloy consisting of the Al--Si alloy with 5 to 1% of copper (Cu) being added thereto as preventive measures for stress migration.
Dry etching of the Al-based metallization layer is carried out generally by using a chlorine based gas. A BCl.sub.3 /Cl.sub.2 mixed gas, disclosed in the Japanese Patent KOKOKU (Publication of Examined Patent Application) Serial No.59-22374, is a typical example. In etching the Al-based metallization layer, a chemical species contributing as a main etchant is chlorine radicals (Cl*) , which are voluntary and cause an etching reaction proceed very quickly. However, the use of Cl* alone results isotropic proceedings of the etching. For this reason, a conventional practice is to cause an ion-assisted reaction to proceed with the incident ion energy being raised to a certain level, and to use decomposition products of a resist mask sputtered by the incident ions as sidewall protection films, thereby achieving high anisotropy. BCl.sub.3, which is a compound added to reduce a natural oxide film on the surface of the Al-based metallization layer, also plays an important incident ion supplying BCl.sub.x.sup.+.
Meanwhile, in the process of sputtering the resist mask by using the incident ion energy which is raised to some extent for securing anisotropy, as described above, a problem of a reduction in the resist selectivity necessarily arises. A selectivity in a typical process is approximately only 2. Such low selectivity generates a dimensional transformation difference from the resist mask in processing a minute metallization pattern, and causes a deterioration of an anisotropic shape.
On the other hand, highly minute design rules of the semiconductor devices demand that the film thickness of a resist coating film be made thin for improvement of the resolution in photolithography. Accordingly, it is becoming difficult to achieve both the high resolution based on the thin resist coating film, and etching with high accuracy via the resist mask formed of the resist coating film.
In order to deal with this problem, deposition of the reaction products onto the resist mask has been proposed so far.
For instance, a process using SiCl.sub.4 as an etching gas is reported in the Extended Abstract of the 33rd Integrated Circuit Symposium, 1987, p. 115. The process is aimed at raising etching durability of a resist mask by covering the surface of the resist mask with CCl.sub.x having low vapor pressure.
Also, a process using BBr.sub.3 is reported in the Proceedings of the 11th Symposium on Dry Process, p. 45, II-2 (1989). The process is aimed at further raising etching durability of a resist mask by covering the surface of the resist mask with CBr.sub.x having a lower vapor pressure than CCl.sub.x. Details of the protection mechanism of the resist mask by using CBr.sub.x are described in the Monthly Journal Semiconductor World, December, 1990, p. 103 to 107, published by the Press Journal Inc. , in which the resist selectivity of about 5 is reported.
However, for achieving resist selectivity of the above-mentioned level, it is necessary to deposit a considerable amount of CCl.sub.x or CBr.sub.x. Therefore, there is a high possibility of deteriorating the particle level in actual production lines.